1. Field of the Invention
The present invention relates to a rotation-angle detection device, an image processing apparatus, and a rotation-angle detection method.
2. Description of the Related Art
In image processing apparatuses, such as printers, facsimile machines, copiers, scanners, or multifunction peripherals, a driving mechanism is driven by a drive motor so that each of necessary units is driven at a predetermined speed for image processing, such as an image reading operation or image forming operation.
Conventionally, in order to drive and rotate a rotator, such as a drive motor, at an intended speed, a sensor is provided around the rotator so as to output a signal that changes in a sine-wave pattern in accordance with the rotation angle of the rotator, and a rotation-angle detection device detects the rotation angle of the rotator on the basis of the output signal of the sensor. Specifically, a magnetic sensor, such as a Hall element, is used as the sensor, and the rotation-angle detection device detects the rotation angle of the rotator by using a sequential search algorithm on the basis of the signal output from the magnetic sensor.
However, in conventional rotation-angle detection devices that use magnetic sensors, errors occur in the sensor output signals due to the difference between the sensitivities of the magnetic sensors used in the rotation-angle detection device, deviation of the phase angle due to the error in the assembly of the magnetic sensor in the rotator, and deviation of the magnetization of magnets.
Therefore, in highly-accurate detection of the rotation angle of the rotator, a calibration operation needs to be performed to correct sensor output signals during a manufacturing process in order to detect the rotation angle of the rotator with a high accuracy.
A conventional rotation angle sensor is proposed (see Japanese Patent Application Laid-open No. 2004-325140) which includes two magnetic sensors that are provided at a predetermined angle with respect to the rotation central axis of a rotator and that output signals in accordance with the strength of the magnetic field; a magnet that generates the magnetic field of strength corresponds to the rotation of the rotator at the positions where the magnetic sensors are located; and a signal processing circuit that processes the output signals from the two magnetic sensors so as to determine the rotation angle of the rotator, wherein the signal processing circuit includes a storage unit that stores Fourier coefficients that is obtained during Fourier series expansion, with respect to the rotation angle, on the vector that is represented by the output values of the output signals that are measured while the rotator is rotated by a predetermined rotation angle; and a calculation unit that calculates the rotation angle by using the Fourier coefficients stored in the storage unit.
In other words, in this conventional technique, after the rotation-angle detection device is assembled in the rotator, it is connected to an external calibration device and the rotator is rotated to measure the output signal of the magnetic sensor within the angle detection device. Then, the Fourier coefficient obtained during Fourier series expansion of the measured sensor output signal with respect to the rotation angle is stored in the storage unit of the rotation-angle detection device as a correction value during calculation of the rotation angle. The rotation-angle detection device detects the rotation angle by correcting the sensor output signal during a rotation-angle detection operation on the basis of the correction value.
However, the conventional technique described in Japanese Patent Application Laid-open No. 2004-325140 has a problem in that, as the calibration operation is performed after the rotation-angle detection device is assembled, the costs are increased due to a calibration process required after the rotation-angle detection device is assembled.
Thus, there is a need to accurately detect the rotation angle of a rotary shaft at low costs without performing an operation to calibrate output signals after assembly.